Apparatus and method for presenting audio in a video teleconference

ABSTRACT

An advanced video teleconferencing (AVTC) system uniquely combines a number of features to promote a realistic “same room” experience for meeting participants. These features include an autodirector to select audio and video sources and to compose shots, a collaboration interface for each participant to communicate nonverbal information, directional LEDs to privately alert participants, audio reflected from the main display, and a collaborative table to share a view of objects or papers on a table. When implemented with sufficient bandwidth for take advantage of these features and to keep latency time low, this AVTC system results in a highly realistic and productive teleconferencing experience.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/463,578, filed Aug. 9, 2006, which is a continuation of U.S. patentapplication Ser. No. 10/911,079, now U.S. Pat. No. 7,092,002, filed Aug.3, 2004, which claims benefit of U.S. provisional patent applicationSer. No. 60/504,085, filed Sep. 19, 2003, all of which applications areincorporated herein in their entirety by this reference thereto.

BACKGROUND

1. Technical Field

The invention relates to teleconferencing. More particularly, theinvention relates to methods for better facilitating collaboration inteleconferences.

2. Description of the Prior Art

Current video conferencing systems are deficient in their ability tosupport effective collaboration among conference participants,engendering a feeling of remoteness amongst the participants that iscontrary to the intended benefits of the system. This deficiency can beattributed to a number of shortcomings that collectively diminish theeffectiveness of the communication.

Specifically, current systems do not: Provide sufficiently low roundtrip latencies; Provide accurate audio cueing, i.e. remote participantvoices often emanate from a location other than where the image of theremote participant is displayed; Allow participants to discuss andnaturally interact with physical objects present at only one location;React to cues provided within speech patterns and content; Adapt thebehavior of the system in response to frequently encountered meetingdynamics; Meaningfully track or consider personal information about theparticipants; or Offer a natural sense of eye contact betweenparticipants.

In conferences involving larger numbers of participants, several otherdeficiencies become apparent. Most notably, current systems do not:Allow participants to receive personalized information withoutdisturbing other conference participants; Provide a natural means forparticipants to track who is present within the conference; or Provide asense of the objects and people present and events transpiring in thefacility surrounding the participants.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts an overview of a VTC installation with five stations,according to the invention;

FIG. 2 depicts the reflection of audio from the main display towards theparticipants, according to the invention;

FIG. 3 depicts an overhead view of the portion of the conference tableonto which the collaborative table is projected, according to theinvention;

FIG. 4 depicts the note passing software which is part of thecollaboration interface, according to the invention; and

FIG. 5 depicts the shared application feature which is part of thecollaboration interface, according to the invention.

SUMMARY

The invention is an advanced video teleconferencing system that allowsan engaging and realistic video conferencing experience. The inventionprovides key visual, audio, security, storage, and administrativeservices needed for a video conferencing experience that cannot beattained with today's conventional methods. These include: anautodirector that automatically selects videocamera shots based on audioactivity and other criteria, which are adjustable with user-settableoperating modes; an interface to allow each participant to communicatenonverbally and privately with any other participants; directionallyaccurate audio reflected from the main display; a collaborative table toallow interaction with 2-D and 3-D objects; and directional LEDs toprivately alert participants. This functionality is provided throughinterfaces that are simple, intuitive, and easily operated by those withlittle or no computer literacy.

DETAILED DESCRIPTION

The herein disclosed advanced video teleconferencing (AVTC) systemprovides a same room experience for meeting participants at physicallyseparated locations, thereby greatly enhancing user interaction andcollaboration. This is met through the combination of a number offeatures, each of which is described in detail below: Low latency toenhance the immediacy of the videoconference experience. An autodirectorto automatically select audio and video sources and to compose shots,based on a number of criteria. These criteria include audio activity,operating modes, participant identification, and other factors.Participant transponders or login to incorporate personal informationinto the autodirector criteria, including relative importance of theparticipant and security clearance. A collaboration interface forparticipants to communicate nonverbal information to any otherparticipants. Directional LED notification to privately alertparticipants. Audio reflected from the main display, such that audioappears to originate from the speakers on the screen. Image-basedparticipant identification for easy reference by participants. Acollaborative table to share a view of objects or papers on a table,along with hand gestures.

For the purposes of this description, a “site” is a single physicallocation at which a VTC system is situated and a “station” is a locationat a site designed to accommodate a single participant with video andaudio pickups and other tools. Each site has a minimum of one station.

FIG. 1 depicts an overview of a VTC site with five stations, accordingto the invention. Each station 10 accommodates one participant, and hasa collaboration interface 11. In the preferred embodiment, the interfaceis a touchscreen via which the participant can send and receive notes,annotate files and images, and perform other tasks. The stations arearranged around a semicircular table 12, which helps facilitates theillusion that participants from other sites are seated around the sametable. Other sites and participants are shown on the main display 13.Supplemental displays 14 can show additional information, such as maps,stills, or moving images.

Narrow field videocameras 15 are aimed at each participant. Adirectional LED is mounted atop each videocamera to privately alert thecorresponding participants. Wide field videocameras 16 capture the roomcontext and can be panned or tilted. Audio is received throughmicrophones for each participant, as well as room microphones to pick upconversation from observers not seated at stations and other ambientsounds. A speaker 17 below the surface of the table is aimed at the maindisplay, thus reflecting audio back to the participants to give theillusion that the audio originates from the display itself.

The collaborative table display 18 is projected on the table 12 from anoverhead projector (not shown), or alternatively from a projectorbeneath the table. The collaborative table is used to show maps andother flat images projected to or from another site, and shows anyobjects held over the area, as well as the hands of the participantpointing out features or manipulating objects.

Adequate Bandwidth for Reduced Latency

Traditionally, there is a perceived tradeoff in video conferencingsystems between available bandwidth and video quality. Given anavailable bandwidth, the image resolution and frame rate are selectedsuch that an aggressive, efficient compression scheme can transmit thevideo signal within that bandwidth. However, experience has shown thatlow bandwidth systems with aggressive and efficient compression anddecompression (CODEC) schemes still yield poor video conferencingexperiences.

The AVTC system is predicated in part on the belief that latency is theprimary cause for this shortcoming. Accordingly, the AVTC systemconsiders a tradeoff between available bandwidth and latency. In thepreferred embodiment, adequate bandwidth is provided to yield round triplatencies well underneath 100 ms, preferably as low as 30 ms. Because alarge fraction of the round trip latency is attributable to theoperation of the CODECs, this essentially requires bandwidth greatenough that the desired image size and frame rate can be transmitted atthe compression ratio achievable by the CODECs in the desired latency.

If such bandwidth is not available, the latency may be preserved byreducing the frame rate or image resolution. Preferably, this tradeoffis adjusted dynamically by the AVTC system. Alternatively it may beadjusted manually, preferably by a meeting administrator seated at anadministrator's console. The ability to adjust this tradeoff may beparticularly useful in network configurations, where the bandwidth usedby the AVTC system is a shared resource.

Autodirector

The AVTC system incorporates an autodirector that emulates the behaviorof a highly experienced human video director. The autodirectorautomatically selects, from among one or more videocamera feeds andother video inputs, a video signal for transmission to remote videoconferencing sites. In this manner, the flow of conversation among theparticipants of a local video conferencing site is seamlesslyreconstructed at the remote teleconferencing site for viewing by theremote participants.

The autodirector selection algorithm has been described in detail in arelated U.S. patent, “Method and Apparatus for Selection of Signals in aTeleconference”, patent Ser. No. 10/327,368, filed Dec. 20, 2002 andallowed Jun. 12, 2004. The description below is focused on two otherunique features of the autodirector, the operating modes and speechrecognition.

Operating Modes

Because the social dynamics of any conference can change as the meetingprogresses, the autodirector incorporates a sense of meeting dynamics indetermining its selection of video inputs. Specific modes can beselected to match particular situations, which will adjust video andaudio selection criteria used by the autodirector. In the case where aparticipant wishes to address and be viewed by all participants of ameeting, the autodirector is configured to force all meetingparticipants to view the participant making the address. This mode istermed speech mode. A related mode is termed lecture mode, and handlessituations where a participant wishes to address another singleparticipant to make an important point. The system is thereby configuredfor a one-on-one exchange between participants. The two participants inthe exchange see each other, and participants at other sites see bothindividuals alternately, according to the dynamics of the verbalexchange. Another mode offered by the autodirector is view force mode,where an administrator at one site can force the selection of aparticular shot from a specified site. The autodirector continues tonormally switch between sites, but whenever the specified site isselected, all other sites view only the particular shot from that site.Finally, in introductory fast switching mode, a rapid switchingalgorithm is provided that allows the system to track the rapidexchanges that typically occur in the initial phase of a conference, inwhich individuals introduce themselves and meet one another.

The autodirector can be placed in any of these modes by a participantlocated at any site, via an administrator console. Priority is given tothe latest received mode request. The current mode is displayed on theadministrator console at each site. An administrator can create newmodes or modify existing modes.

If the administrator does not specify a mode, the autodirectorpreferably selects the appropriate mode based on assumed or detectedmeeting dynamics. For example, in a typical business meeting,conversation often begins with a series of introductions amongparticipants, often as a sequence of rapidly changing speakers. Tocapture these events effectively, the audio and video signal selectedshould also be changed rapidly. The autodirector may thereforeautomatically enter introductory fast switching mode as a meetingbegins. Thus, the autodirector, aware of introductions as part oftypical meeting dynamics, initially allows signal switching patternsthat might appear abrupt or hurried under other circumstances.

Later in a typical meeting, meeting dialogue becomes more focused anddirected. The duration for which an individual speaks typicallyincreases as more complex issues are addressed, and extendedpresentations may be made. The autodirector may therefore readily enterspeech mode and lecture mode during the middle of a typical meeting. Anappropriate time to enter into these modes may be detected by analyzingspeech patterns.

The AVTC system also stores specialized sequences of modes for use witha particular style of meeting. For example, one sequence of modes may beselected for use during a short staff meeting, while a differentsequence of modes may be used for an extended board of directorsmeeting. These sequences are fully customizable and are stored withinthe system for later access. The system can therefore be made aware ofdynamics specific to a particular group of participants.

The resulting system is an automated video conferencing direction systemthat functions in accord with, rather than in spite of, typical meetingdynamics.

Speech Recognition

To provide an enhanced understanding of which video signal isappropriate for selection at a particular time, the autodirectoranalyzes the conference audio. In one embodiment, the autodirectorfavors a shot of a participant when his or her name is detected on theaudio; this is effective at capturing reaction shots. Limiting speechrecognition processing to searching the audio for participant namesgreatly reduces the complexity of this problem.

More generally, specific words or phrases may be recognized and treatedas attractors or pointers. Attractors increase the likelihood ofselecting a video signal containing an image of the participant whouttered the attractor word or phrase, such as “I think,” “In myopinion,” or “I have here”. Pointers, of which participants names areone example, encourage the selection of a particular participant otherthan the participant who uttered the phrase.

Audio Reflected from Monitor for Improved Audio Directionality

When audio does not seem to be coming directly from the same directionas the main display, such as from side speakers, fatigue can begenerated among the participants. Conversely, proper audiodirectionality results in more natural, effortless communication betweenparticipants.

To provide accurate audio directionality, the AVTC system incorporatesaudio speakers directed at the main display showing images of the remoteparticipants. Audio emanating from the speakers is reflected from thedisplay and towards the listening participants, thus appearing toemanate directly from the participant shown on the main display. Thiseffect is further enhanced by selecting a highly directional audiospeaker and shielding the speaker in a manner that blocks the straightline transmission pathways between the speaker and the listeningparticipants.

FIG. 2 depicts the reflection of audio from the main display towards theparticipants according to the invention. The speaker 21 is mountedbeneath the surface of the table under an acoustically transparentmaterial 22 and aimed at the main display 12. Audio is thereby reflectedfrom the main display towards the participants 23, giving the illusionthat the audio is originating from the center of the display.

Participant Transponders

To obtain and meaningfully incorporate participant personal informationinto the conference environment, one embodiment of the AVTC system usesparticipant transponders. Each transponder provides information about avideo conferencing participant, such as his location within theconference site, organizational position and title, clearance level, andspeaking characteristics. The information is provided to theautodirector, which can then make more intelligent decisions based onthe personal information.

The participant transponder is worn or carried by the video conferencingparticipant, for example in the form of a radio frequency identification(RFID) tag embedded in a personnel ID card. More generally, the devicemay be incorporated within a badge, pen, laser pointer, wirelessmicrophone, or other device common to a video conference setting. In thepreferred configuration of the AVTC system, the transponder isintegrated within a station at the conferencing site through the use ofexisting RFID card technology, either passive or active. In thisscenario, the participant carries an RFID card, and the card reader isincorporated within the station, which is in turn connected to the AVTCsystem.

Regardless of the form factor or communication mechanism, thetransponder need not broadcast all information about the participant,but instead can broadcast a participant identity that allows the AVTCsystem to access a complete participant profile within a database. Fromthis database the autodirector can determine a participant hierarchy, aswell as a security level for the conference as a whole. The conferencesecurity level is displayed for all participants by the autodirector.The autodirector can accept changes to the hierarchy, allowingparticipants to take on varying priority levels as a meeting evolves.

The database may also contain biometric data for the participants. Thisallows the identity of the participant to be verified by biometricmeasurements, such as facial features (e.g. as captured by one or moreof the video cameras), or fingerprint or retinal scans.

The transponder may also allow the location of the wearer within theconference room to be determined. This can be performed locally by thetransponder, and then broadcast to the AVTC system when queried.Alternatively, the location can be determined through time of flight ordirectional triangulation calculations based on measurements acquiredfrom sensors placed throughout the conferencing site and operated by theAVTC system. The transponders may also be used to detect the entry orexit of a participant by analyzing the positions of the transpondersignals it receives. In particular, the security level of the meeting isadjusted to reflect the participants currently present. Based on thecurrent security level, the AVTC system allows or prohibit access tospecific information or services.

Collaboration Interface

Each station is fitted with a collaboration interface for sendingnonverbal information to other participants. The interface consists of apen-enabled touchscreen display, preferably to one side of the stationso as not to block the view of the participant. The interface providesaccess to a suite of collaboration tools: Login—If participanttransponders are not used, the participants can use this screen to loginor can run an identification badge through a card reader at the side ofthe interface. The system can then display the person's name and titleto other participants. Personalized services and preferences can be setfor specific participants, such as directing their important email totheir interface. When the participant logs out from the conference, theautodirector can cease selecting the view of that station.Alternatively, a presence indicator can detect if a person is sitting atthe station. Note sending—Each participant can use a virtual whiteboardto send notes or hand-drawn illustrations to any or all participants,who are alerted by a directional LED mounted above each station'svideocamera. The contents of the whiteboard can be printed on a localprinter, or cleared by a “Clear” button.

FIG. 4 depicts the note passing software which is part of thecollaboration interface, according to the invention. The note passingmode is selected from tabs 40 at the top of the interface. Notes orsketches can be handwritten on the notes area 41 using an electronic pentool or similar device (not shown). To send a note, the participantselects another participant from the image-based identification area 42,which contains thumbnail images of each participant. The note can thenbe sent or printed through control buttons 43. Note recipients areprivately alerted by the flashing of a directional LED aimed at theirstation. Web browser—Any participant can view, annotate, or send Webpages to other participants. Shared applications—Participants canoperate software applications, such as Microsoft PowerPointpresentations, on their interface with the same annotation anddistribution capabilities described above. FIG. 5 depicts the sharedapplication feature which is part of the collaboration interface,according to the invention. In this depiction, the shared application isslide presentation software. Participants can send any image or slide 50to the collaboration screen. All participants see the contents of thecollaboration screen, which is akin to an electronic whiteboard.Participants can handwrite annotations 51, which appear in differentcolor inks to distinguish participant's annotations.

Image-Based Participant Identification

To provide identification of the participants within a conference, theAVTC uses an image-based approach. A still image of a participant iscaptured using one of the several video cameras present. Preferably, theimage is captured by the camera positioned to capture a close up shot ofthe participant. The image is then reduced in size and converted to asuitable format for presentation on a computer display, for exampleTIFF, GIF, or JPEG. The image may then be presented on the displays ofother users as an iconic representation of the presence of theparticipant in the meeting. For example, the image may be presented atthe remote location among a set of pictures indicating who is present atthe local location. Participants can indicate the recipients of itemssuch as emails and notes by selecting the image of other participants.FIG. 4 depicts an embodiment of participant images 42.

Alternatively, images of the participants may be retrieved from adatabase of user information, based on a username obtained fromparticipant transponders or by the login or ID card reader mechanismsdescribed herein. Alternatively, the image may be stored directly on theID card.

Regardless of the technique used to obtain the image of the participant,by presenting images in addition to names, the AVTC system provides anatural way for one participant to determine other participantscurrently present. Because many individuals find faces easier torecognize and remember than names, this approach allows participants tointeract with one another in a more comfortable manner.

Directional LED Participant Notification

During video conferences with multiple participants at a single site, itis often desirable to obtain the attention of a single participantwithout disturbing the other participants. For example, the AVTC systemmay wish to notify a participant that he is needed in another meeting,that new email has arrived, or that a note has been received via thecollaborative interface.

To provide such notification, the AVTC system incorporates a directionalLED-based cueing device. An LED is positioned at the end of a relativelylong tube, with the axis of LED illumination directed along the lengthof the tube. The axis of the tube is aligned with the participant to bealerted so that when the LED is illuminated, it is visible only to theintended participant. The length of the tube is selected based on thespacing between participants and the distance from the tube to theparticipants.

Preferably, the system provides one notification device for eachparticipant. Alternatively, a single tube and LED may be actuated, suchthat it may be aligned with a particular participant to be notified.Notably, the preferred embodiment offers the advantage of simultaneousnotification of more than one participant. Further, the LED and tube arepreferably positioned near the main display, so that the notificationdevice is usually within the field of view of the participant. In oneembodiment, the tubes are mounted along the optical axis of the camerasaimed at each participant. The notification device can then furtherfunction as an alignment device, wherein each participant may adjust hisposition until he or she is aligned with the tube, ensuring a properlycomposed individual shot.

Alternatively, an alert may be presented on a personal display locatedalongside the participant. However, because the primary focus of theparticipant is on the main display, LED-based notification is preferred,because it more likely to be immediately noted by the participant.

Camera Coverage of Secondary Participants

Since wide views do not give sufficient participant details, most shotsin a videoconference are close-ups. However, these do not give remoteparticipants a feel for the surrounding conference room environment andof secondary participants.

To address this problem, the AVTC includes one or more room microphonesand cameras that provide audio and camera coverage of secondaryparticipants not positioned at stations. For example, if the primaryparticipants are seated at a conference table centered in front of amonitor positioned flush with one wall of a conference room, the roomcameras provide coverage of the corners of the room on the side oppositethe monitor.

The video and audio signals provided by the room cameras and microphonesare provided to the autodirector. The autodirector then determinesappropriate times to transmit these video and audio feeds to the remotelocation, providing a sense of context to the remote participants. Forexample, the room camera feed may be selected when the primaryparticipants are quiet, when a secondary participant speaks for anextended period of time, or if the entry or exit of a participantthrough a conference room doorway is detected.

Collaborative Table

The AVTC provides participants with a collaborative table upon whichobjects and hand gestures can be viewed at several sites.

For convenience, the table at one site is termed the source table, andthe table at a second site is termed the viewing table. A participantsits at the source table and places objects or documents of interestupon it. Above the source table is a high definition table videocamerawhich captures images of the objects, which are encoded and transmitted.At the other site or sites, a projector, preferably high-resolution andoverhead mounted, projects the images onto the viewing table.

FIG. 3 depicts an overhead view of the portion of the conference tableonto which the collaborative table is projected, according to theinvention. The image 18 is projected on the conference table 12 from aprojector overhead from another video teleconference site, and iscentered in front of the participant 20 situated at the middle of theconference table. The high definition image shows both still objects inthe background 21 and the hands of a participant at the other sitemanipulating a solid object 22.

If the source participant gestures with his hands over the source table,then the participant at the viewing table sees the source participant'shands. Given limited bandwidth, this leads to a seemingly conflictingrequirements whereas objects of interest should be presented at themaximum possible quality (low frame rate), yet the hand gestures shouldappear smooth and fluid (high frame rate). In addition, the participantat the viewing table should in turn be able to make hand gestures at theprojected objects that are in turn sent and projected onto the sourcetable.

Encoding

Described below are three embodiments of an encoding scheme for thecollaborative table which provide for both high resolution, largelystatic scenes, with lower resolution but high frame rate hand gesturesand motion. These approaches range from an computationally lessexpensive but lower quality method (motion detection) to a high qualitycustom mechanism.

Motion Detection with High and Low Frame Rate Encodings

Open source MJPEG code is used to generate a motion JPEG RTP stream withtight custom encoding. MJPEG allows different image resolutions in eachframe, and each frame is JPEG encoded with no intraframe dependencies.The system constantly captures images and detects significant changesbetween frames. If no significant change has occurred over a period oftime, then the system enters static mode, and sends a high quality JPEGframe onto the channel via RTP. A single high quality frame takesseveral seconds to send at T1 bandwidths, and is sent about every 30seconds so that any new listeners are updated in a timely manner.

If the system detects a significant change in the frame above a certainthreshold, it immediately switches to dynamic mode, encoding high framerate, lower resolution images into the RTP MJPEG stream. Listeningclients automatically adjust to the new frame resolution. Because thesystem is MJPEG compliant, it can use an existing RTP/MJPEG client withlittle or no modification. If new motion is not detected after severalseconds, the system switches back to static mode and starts sending highquality, very low rate frames.

A disadvantage of this technique is that if, for example, the sourcetable consists of a map over which someone is gesturing, when the persongestures over part of the image with their hands the entire image,including the map, is degraded to illegible quality until the personremoves their hand or holds their hand very still.

Masked Hybrid: High and Low Frame Rate Streams with Masking

In dynamic mode, this technique uses a new RTP channel called the maskchannel, containing low resolution run-length-encoded bit-masks tocorrespond to each of the transmitted high frame-rate, low quality MJPEGdynamic frames. When the source system enters dynamic mode, it retainsthe last transmitted static high quality frame as a reference image andcompares all successive dynamic frames to a scaled down reference frame.It computes and thresholds the pixel-by-pixel change between thereference frame, scaled to the dynamic mode resolution, and each dynamicframe and builds a bit-mask of changed areas. This bit-mask is labeledand transmitted on the mask channel after the dynamic frame is sent.Each RTP display client receives the mask and the dynamic frame, andcorrelates the two according to the label. The client then copies thehigh resolution reference frame, and overlays the scaled-up dynamicframe according to the mask.

This technique allows stationary objects on the source table to retaintheir full-resolution appearance while low fidelity, high frame ratehand gestures are overlaid in a lower resolution.

Custom Encoding

A higher quality solution is obtained with an entirely custom codingstrategy. The high resolution scene is decomposed into a multiscaleimage, composed of a stack of images with the lowest quality base imageoverlaid by successive images. These images contain higher order termsrefining the quality of the base image up to any level of desiredquality, very similar to a wavelet deconstruction.

The base image is small enough to allow a full frame, high ratetransmission in any circumstance, e.g. 135.times.238.times.2 bitgreyscale. Higher order images successively increase depth andresolution in scale space up to full high-definition, 1080i resolution.For a transmission model, the images are subdivided into about 1024spatial blocks.

To create the multiscale image, starting at the base layer the encodercomputes a difference value from the prior frame for each spatial block.If the difference value for a block at that layer is significant, thanthe encoder flags the block and all corresponding higher layer imageblocks as needing to be retransmitted with a certain disparity score.The encoder then starts at the base layer and transmits via RTP eachchanged block in that layer in the order of it's disparity score andmarks the transmitted blocks as current.

If there is time before the next frame, the encoder then goes to thesecond order image in the stack and transmits any flagged blocks untilit either runs out of bandwidth before the next frame, or runs out offlagged blocks on that layer image. If there is time before the nextframe, then the encoder traverses higher and higher layer images andtransmits changed blocks, dynamically capturing more and more imagedetail.

If the scene becomes static, this transmission algorithm automaticallybuilds and transmits a high quality representation of the image overseveral seconds because small changes such as lighting and camera noise,should not affect the lower layer base images. This gives the encodertime to transmit blocks from the detailed higher order images.

In the case of a largely static scene with a hand moving over it,performance is boosted by comparing each block not just with the priorframe, but also with a reference frame that has been built up over time.If a hand is removed from over a portion of a static scene, and thatportion of the image corresponds well with the reference image, thenthose blocks can be flagged to revert to their detailed reference stateinstead of being queued for full retransmission. Each client maintainsthe shared multiscale representation, adjusting it as new blocks or“revert block to reference state” flags are sent.

This algorithm provides optimal performance for the stated requirements,providing high frame rate when change is prevalent, but at the maximumallowable quality given the amount of change in the scene, whilepreserving a high quality reference state.

Simultaneous Projection and Image Capture

Challenges arise when simultaneously capturing and projecting video ontothe collaborative table. For example, a map on the source table isprojected onto to the viewing table, while simultaneously the hands ofthe participant at the viewing table (pointing at the projected map) areprojected onto the source table. The problem is that not only the mapimage, but also the hands of the participant at the viewing tableprojected onto the source table, are transmitted back to the viewingtable.

In theory, both the actual map on the source table and the projectedhands could be captured, subtract the projected hands, then transmit themap image without the hands. In reality, this is a nontrivial if notincomplete problem, especially when the projection includethree-dimensional items of unknown shape, varying reflectivity, andunknown color.

Instead, the system momentarily blanks the projector to black when thecamera is capturing a frame, and runs both the table videocamera and theprojector at a reduced duty cycle and frame rate. By synchronizing thecamera capture, only 30 ms are needed to capture a frame. Becauseencoding and transmitting at full frame rate may be impractical for thereasons described earlier, capture is preferably executed at 5frames/sec by blanking the projector 5 times a second for 30 ms eachtime. This blanking is generally imperceptible to the viewer of theprojection.

Projector Blanking Issues

Implementing the projector blanking solution presents two practicalproblems: Rapid and precise blanking—Successfully blanking the projectorby interrupting the video to it for a period of a video frame requires arapid response from the projector. While LCD projectors typically havesomewhat slow response times, a broadcast frame is considerably longerthan a typical high resolution graphics frame, so an LCD projector maybe suitable. However, using a higher quality CRT-based projector ispreferable due to their faster response times.

For the software driving the projector, swapping the video output to ablank buffer for a few frames and outputting a synchronization triggersignal for the camera can be readily achieved with contemporary consumergraphics cards. If the image buffer is to be redrawn while the blankbuffer is being displayed, the double buffering functionality supportedby the majority of graphics cards may be used.

Another approach to blanking the projector without concern for theprojector response time incorporates an LCD shutter over the projectorlens. This reduces the gain of the projection, but existing stereovision LCD shutter glasses demonstrate that LCD shutter response time isfar more than adequate for a broadcast frame blanking. Synchronizedblanking—Synchronization of the blanking with image capture is achievedusing an external sync signal. However the sync signal is generated,e.g. by the camera or by an external source, the signal is input into acounter to signal the computer to blank the projection periodically andcapture the next frame, with suitable projector refresh timing.Latencies in the capture hardware may complicate this process slightly.Because the horizontal refresh rate for the projector is likely to betwo or three times the broadcast frame capture rate, delay due toprojector frame rate should be minimal. Regardless, with a fast enoughprojector, the timing problem is readily addressed.

An alternate embodiment polarizes the light from the projector, and usea polarizing filter on the camera to filter out the polarized projectedimage, which may be preferable to the more complex projector blankingand camera synchronization solution.

Background Segmentation

At the viewing table, the system simultaneously-captures the viewer'shand gesturing with the camera while projecting the source table'simages onto the table. The same projector/camera synchronizationdescribed above is used to separate the hands from the projection. Thebackground for the viewing table is presumed to be white or some othersolid color. However, an additional problem is encountered at theviewing end, namely that only the viewer's hands should be projecteddown onto the source table, without adding a lot of light and flicker byprojecting the white background onto the source table.

Thus, the viewer's hands are masked from the (presumably) whitebackground, and replaced with a black mask. This is easily accomplishedthrough simple thresholding. If the background is a color other thanwhite, color segmentation and connected component labeling can beperformed on the viewer side before encoding of the hand video. If thebackground is not a solid color, background segmentation is slightlymore complicated, but may be performed with the thresholded backgroundmasking described herein in connection with encoding.

Camera-Projector Calibration

To ensure proper functioning of the hand overlays from the viewing tableto the source table, it is necessary to calibrate the relative positionand scale of the projected image and the camera capturing the handmotion on the viewing table. This calibration may be automated byprojecting a set of patterns onto the table and detecting their positionin the camera with simple image processing. Merely projecting a solidscreen and detecting the corners provides most of the necessaryparameters.

Although the invention is described herein with reference to severalembodiments, including the preferred embodiment, one skilled in the artwill readily appreciate that other applications may be substituted forthose set forth herein without departing from the spirit and scope ofthe invention.

Accordingly, the invention should only be limited by the followingclaims.

1. A video teleconferencing system comprising: a first display screenwithin view of each participant of a video conference; at least onevideocamera paired with said each participant of said video conference;at least one microphone paired with said each participant of said videoconference; a video switching system that automatically selects a singleincoming video signal originating from one of said at least onevideocamera for display on said first display screen, based on at leastone of: an audio activity level of at least one of said microphones; andselection parameters specified by an active operating mode among aplurality of available operating modes.
 2. The system of claim 1,wherein said active operating mode is specified by at least one of saidparticipants.
 3. The system of claim 1, wherein said active operatingmode is automatically selected by said automated video switching system.4. The system of claim 1, wherein said selection parameters can beadjusted by at least one of said participants.
 5. The system of claim 1,wherein selection parameters for additional available operating modescan be specified by an administrator of said system.
 6. The system ofclaim 1, wherein said automated video switching system stores sequencesof said active operating modes, each of said sequences reflecting aparticular meeting dynamic.
 7. The system of claim 1, wherein saidavailable operating modes comprise a speech mode wherein said selectionparameters ensure that a video signal among said video signals thatdisplays an image of a specified participant among said participants ata first site is displayed continuously to all participants at othersites.
 8. The system of claim 1, wherein said available operating modescomprise a lecture mode wherein said selection parameters ensure thattwo specified participants among said participants view only videosignals among said video signals displaying images of each other, andother participants among said participants alternately view videosignals among said video signals displaying first one of said twospecified participants and then a second one of said two specifiedparticipants.
 9. The system of claim 1, wherein said available operatingmodes comprise a view force mode wherein said selection parametersensure that said automated video switching system at a first of said atleast one site selects for transmission a video signal specified by oneof said participants at a second of said at least one site.
 10. Thesystem of claim 1, wherein said available operating modes comprise afast switching mode wherein said selection parameters ensure that saidautomated video switching system selects from among said video signalsat an increased frequency, so that said automated video switching systemtracks rapid exchanges between said participants.
 11. The system ofclaim 1, further comprising a transmitting system for transmitting saidsignal originating from one of said at least one videocamera to any ofsaid video switching system and any of said first display screen withinview of each participant.